Originally Posted by Unregistered

how do you guys decide when to spend FG for the gacha? I've got 112 FG burning a hole right now, and I typically base it on if the 6* that's being offered is something i want. Only issue is, I can't tell which 6* are good and worth getting, and which 6* are a step down. Anyone have advice?

All 6* are an improvement over 5*, except Alstroe Maria, who is more or less a 5* disguised as a 6*. So first rule (the waifu rule) is always: pick the girls you like simply based on their art.

It's only when you set up teams around synergies or stacking buffs that replacing a 5* girl with a random 6* might effectively lower performance. In those cases it's advisable to replace a 5* with a 6* that has similar buffs (and possibly, is of the same type/color as well!).
For example, in my counter-squad, I didn't replace Snapdragon for quite a while, even though I had several unused 6*s. But when I finally got the 6* Viola (Yukata), I speed leveled her, since she shares the counter ability and has the same typing, and was a suitable replacement.

The Abilities list is very useful for figuring out the combat benefits of each girl.